Quantcast Section 2. Shipboard Systems

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2.2.1 CHT SYSTEMS.  As previously stated, these systems are being installed
on most of the Navy's large combatant ships.
a. Three Distinct Modes of Operation Accommodated by CHT Systems.
(1) Soil and Waste Transfer to Pier Sewer System. While a ship is
in port, soil and waste drainage is collected in the CHT tank and trans-
ferred/discharged by pumping from the tank to a pier sewer system.
(2) Waste Diverted Overboard. When a ship is transiting
restricted waters, the ship's sewage is collected from the soil drain and
held in the holding tanks.  In this holding mode of operation, waste drainage
usually can be diverted overboard by gravity.
(3) Soil and Waste Diverted Overboard. When the ship is outside
restricted waters, both soil and waste drainage are diverted overboard by
Types of CHT Systems.
Two Basic
(1) Comminutor.  In this type of system, a comminutor has been
incorporated into the inflow drain lines of the holding tanks.  This system is
used for tanks with greater than 2,000 gallons capacity (see Figure 2-l).
(2) Overflow Box and Inflow Strainers. This system is used for
tanks with capacities smaller than 2,000 gallons (see Figure 2-2).
co  Sewage Storage Tanks.
Depending upon the size and class of the ves-
sel, CHT systems will include one or more sewage storage tanks.  Generally,
the CHT tanks are sized to hold up to 12 hours of sewage flow, based upon a
normal ship complement.  However, due to space constraints, many ships do not
meet this design objective.  When a ship is in port, sewage and waste water
are collected in the ship's CHT tank(s), after comminution or screening.  Each
CHT tank is provided with two automatically controlled sewage pumps which
transfer sewage ashore under pressure.  Water level sensors within CHT tanks
control pumping cycles to minimize detention.  A high water level alarm which
registers at a continuously manned remote location is also installed in each
holding tank.
CHT sewage holding tanks are equipped with a diffused air supply to
inhibit sewage septicity and to help keep solids in suspension.  In addition,
each tank is provided with an internal salt water washdown system to permit
cleaning after each period of use.  About 100 gallons per minute (gal/rein) of
salt water is obtained under pressure from the ship's fire system.  The fire
system is also connected to the sewage discharge force main near the outlet of
the sewage pumps to allow salt water flushing of the system's pumps and piping
after use.  This feature is also called upon for salt water flushing of sewage
hoses, as will be explained in Chapter 3, Section 2.


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